1. Field of the Invention
The present invention relates to an apparatus for inspecting a substrate or circuit board on which is arranged two-dimensionally a plurality of terminal pads or contact pads (herein referred to as lands) for electrical connection with a chip or another circuit board (circuit board provided with lands being herein referred to as "land-attached circuit board").
2. Description of the Related Art
In recent years, there is an increasing tendency that integrated circuit chips, such as microprocessor chips and computer chips are becoming higher in the density of integration and rapidly increasing in the number of input/output terminals. Such a chip is connected to a printed circuit board such as a motherboard by way of a flip-chip connecting board. The number of lands formed on the front surface of the connecting board is several hundreds at the minimum and several thousands at the maximum. Further, on the rear surface of the connecting board are formed a number of lands which correspond in position to the respective lands of a matching printed circuit board for connecting the connecting board to the printed circuit board by means of solder balls, etc. In this connection, if the lands formed on the substrate are, for example, not in place or not uniform in the height level of the surface thereof due to waviness, warping, bending, etc., there arises a problem in that a defective connection may possibly occur to cause a malfunction of an integrated circuit, so strict inspection of the lands is necessitated.
Heretofore known as the most popular inspecting technique for inspecting the formed condition of each land of such a land-attached circuit board, is an inspection technique of picking up a rear surface side image of the board by means of a CCD camera or the like and making a judgement on whether the formed condition of each land is good or not, on the basis of the picked-up image. However, such an inspection technique based on the picked-up image has a disadvantage in that it can never obtain information on land surface height distribution though it can carry out inspection of the position, area, etc. of the land with ease. In this connection, it is considered to first carry out measurement of land area or other dimensions on the basis of the picked-up image and then carry out inspection of height distribution by using a known laser height measurement technique such as a knife edge technique and confocal technique. However, such inspection and measurement require a laser height measurement apparatus and an inspection apparatus for inspection on the basis of a picked-up image, respectively and separately, thus resulting in an increased cost of the apparatus. Further, by the known laser height measurement technique, it takes several seconds to measure the surface height level of one land (hereinafter referred to as land height level). Thus, in case of the board having a great number of lands, it takes a time ranging from several tens of minutes to several hours to measure the heights of all the lands on one circuit board, so it is actually impossible to carry out 100% inspection of mass-produced circuit boards.